An all chemical route to design a hybrid battery-type supercapacitor based on ZnCo₂O₄/CdS composite nanostructures

Highlights

• ZnCo₂O₄ electrode is synthesized by hydrothermal technique.

• ZnCo₂O₄/CdS composite is synthesized using chemical route.

• ZnCo₂O₄/CdS exhibited an areal capacitance of 2658 mFcm⁻² at a scan rate of 5 mVs⁻¹.

• An energy density of 517 517 μWhcm⁻² is observed.

• An all chemical route is adopted for the efficient battery-type supercapacitor fabrication.

Abstract

Recently, spinel-type binary metal oxides have attracted enormous interest in energy storage devices. In supercapacitors improving energy density is still challenging task and the composite nanostructures are found to address this issue in some extent. Herein, a composite nanostructure based on ZnCo₂O₄/CdS was synthesized on nickel foam using hydrothermal and successive ionic layer adsorption and reaction (SILAR) methods. A hydrothermally synthesized ZnCo₂O₄ nanoflowers were coated by CdS nanoparticles by varying SILAR cycles and studied its electrochemical performance. The ZnCo₂O₄/CdS nanostructured electrode with optimized four SILAR cycles of CdS coating exhibited a high areal capacity, energy density and power density of 2658 mCcm⁻², 517 μWhcm⁻² and 17.5 mWcm⁻² at 25 mA, which is higher than pristine ZnCo₂O₄. This work show ZnCo₂O₄/CdS nanostructure is a favorable electrode for supercapacitors.

Introduction

The modern technology driven life demands the energy security essential to the day-to-day life. World is facing an enormous and urgent challenge of climate change arisen due to use of fossil fuels to fulfill the global power demand. It is inevitable to shift our attention from non-renewables to green renewable alternatives in the energy dependency. Supercapacitors or ultracapacitors are found to be one of the viable energy storage devices owing to their higher energy density than conventional capacitors and deliver much higher power densities than batteries [[1], [2], [3], [4]]. The kind of charge storage mechanisms have led SCs to be divided into two type's viz., electrical double layer capacitors (EDLCs) and pseudocapacitors. Various carbon derivative materials like graphite, activated carbon, carbon nanotube and graphene behave as EDLC [[5], [6], [7], [8]]. However, the low specific capacitance of carbon based materials hinders their practical application as an energy storage device. Different transition metal oxides (TMOs)/hydroxides, conducting polymers, metal sulfides/selenides, transition metal dichalcogenides (TMDCs) are the class of pseudocapacitor [[9], [10], [11], [12], [13], [14], [15]]. Based on their high merit of specific capacitance pseudocapacitors are superior to EDLCs due to the significant redox properties of the former.

In recent days, it has been researched that use of two different metal oxides to form binary oxide materials has contributed in elevating their redox activities. MnCo₂O₄, NiCo₂O₄ and ZnCo₂O₄ are few examples of binary metal oxides which have established the faster redox reactions [[16], [17], [18], [19], [20]]. Such binary metal oxides are also referred as spinel type metal oxides. In these spinel type of metal oxides the transition metal incorporated spinel structures are found to be best candidates for supercapacitors [21]. ZnCo₂O₄ is gaining recent attention as an electrode material for supercapacitor due to its promising electrochemical properties, high electrical conductivity, environmental friendliness and its abundancy. The Zn²⁺ and Co³⁺ cations present in spinel ZnCo₂O₄ offer enhanced ion transport with high specific capacity and good electrochemical stability [[22], [23], [24], [25]]. However, ZnCo₂O₄ suffers poor electrochemical stability owing to its low intrinsic conductivity and structural instability during charging-discharging process [26]. In order to overcome these drawbacks, nanostructure composite electrodes with high electronic conductivity, optimal diffusion path and having electrochemically stable ability is needed to be designed and fabricated. Transition metal sulfide like CdS is recently attracting as a candidate for forming composite structure with metal oxides or spinel oxides due to its excellent conductivity. CdS semiconductor is having good electrical conductivity and environmental stability. CdS possess high theoretical capacity of 1675 Fg^-1 along with good electronic conductivity and is low cost due its natural abundancy. The literature insight gives that CdS is an excellent candidate for electrochemical supercapacitor despite less explored [[27], [28], [29]]. However, due care must be taken while synthesizing CdS due to toxic cadmium content.

Present work highlights a composite structure based on spinel oxide and binary metal sulfide using hierarchical ZnCo₂O₄/CdS for enhanced pseudocapacitor studies. An all chemical route is employed to fabricate ZnCo₂O₄/CdS composite structure, wherein ZnCo₂O₄ nanostructure is synthesized by facile and one pot hydrothermal technique and CdS nanoparticles are decorated onto it by successive ionic layer adsorption and reaction (SILAR) technique. An integrated ZnCo₂O₄/CdS electrode for SCs exhibited an enhanced specific capacity, superior rate capability and good cyclic stability due to the hierarchical nature of ZnCo₂O₄ and higher electronic conductivity of CdS. This unique spinel oxide and metal sulfide composite structure provides a good alternative for the great prospects of energy storage devices of the future.